1. Field of the Invention
This invention relates to a gas turbine combustor and more specifically to an improved cooling configuration for an interface region between a combustion liner and a transition duct.
2. Description of Related Art
A gas turbine engine typically comprises a multi-stage compressor, which compresses air drawn into the engine to a higher pressure and higher temperature. A majority of this air passes to the combustors, which mixes the compressed heated air with fuel and contains the resulting reaction that generates the hot combustion gases. These gases then pass through a multi-stage turbine, which drives the compressor, before exiting the engine. In land-based gas turbines, the turbine is also coupled to a generator for generating electricity.
For land-based gas turbine engines, often times a plurality of combustors are utilized. Each of the combustion systems include a case that serves as a pressure vessel containing the combustion liner, which is where the high pressure air and gas mix and react to form the hot combustion gases. The hot combustion gases exit the combustion liner and pass through a transition duct, which directs the flow of gases into the turbine. The transition duct is typically surrounded by a plenum of cooling air that exits from the compressor and cools the transition duct prior to being directed towards the combustor inlet for mixing with fuel in the combustion liners. An example of a gas turbine combustor of this configuration is shown in cross section in FIG. 1. Combustor 10 comprises an outer casing 11, a combustion liner 12 located within outer casing 11, and an end cover 13 fixed to outer casing 11, wherein end cover 13 includes a plurality of fuel nozzles 14 for injecting fuel into combustion liner 12. Located between combustion liner 12 and turbine 15 is a transition duct 16, which transfers the hot combustion gases from the combustion liner to the turbine.
In operation, compressed air, which is represented by the arrows in FIG. 1, exits from a compressor into plenum 17 and passes around transition duct 16, cooling the transition duct outer wall 18, before passing between outer casing 11 and combustion liner 12 where it cools combustion liner outer wall 19. Finally the compressed air mixes with fuel from fuel nozzles 14 and combusts inside combustion liner 12.
Due to the high temperatures inherent with the combustion process, it is important to provide sufficient cooling to the combustion hardware in order to maintain its durability. One particular region where this is especially important is the interface between the combustion liner and the transition duct, which is shown in greater detail in FIG. 2. Combustion liner 12 is inserted within transition duct 16, with combustion liner 12 having at least one seal 20 for engagement with transition duct 16. Although seal 20 is designed to prevent large quantities of cooling air from entering transition duct 16 from plenum 17, it is desirable for a controlled amount of cooling air to pass through channel 21 located between combustion liner 12 and transition duct 16 to cool the outer aft end surface of combustion liner 12. Poor cooling at the combustion liner aft end results in higher combustion liner metal temperatures and more interference between seal 20 and transition duct 16 due to larger amounts of thermal growth by liner 12 and seal 20. A greater interference between mating parts results in increased wear to the seal requiring premature replacement.
Another feature found in the aft end of prior art combustion liners is deflector 22, which is a circumferential plate located within combustion liner 12 that is angled inward and deflects hot combustion gases away from the liner aft end region and is intended to reduce the amount of hot combustion gases that would otherwise re-circulate back into channel 21 between the combustion liner and transition duct. By altering the flow path of the hot combustion gases, the flow is also better mixed.
However, the hot gas flow that has been redirected by deflector 22 tends to adversely affect the heat transfer on the transition duct and first stage turbine vanes and increase their metal temperatures, thereby reducing their component life. The large regions of turbulence created by deflector 22 results in some combustion gases inadvertently being re-circulated back into channel 21, thereby blocking the small amount of cooling air currently supplied to the channel. As a result of this re-circulation effect, less cooling of seal 20 occurs and higher metal temperatures for combustion liner 12 and transition duct 16 are present. It has been determined that the primary benefit of the deflector, that is redirecting the hot combustion gas flow away from the combustion liner aft end, is not sufficient enough itself to reduce metal temperatures of the combustion liner aft end and prevent excessive wear to seal 20. Therefore modifications to enhance the cooling effectiveness as well as to eliminate unnecessary regions of high turbulence that contribute to high combustion liner metal temperatures are required.